Organic light-emitting diodes (OLEDs) are a promising technology for flat-panel displays and area illumination lamps and backlights. Applications of OLED devices include active-matrix image displays, passive-matrix image displays, and area-lighting devices such as, for example, selective desktop lighting. Irrespective of the particular OLED device configuration tailored to these broad fields of applications, all OLEDs function on the same general principles. An organic electroluminescent (EL) medium structure is sandwiched between two electrodes. At least one of the electrodes is at least partially light transmissive. These electrodes are commonly referred to as an anode and a cathode in analogy to the terminals of a conventional diode. When an electrical potential is applied between the electrodes so that the anode is connected to the positive terminal of a voltage source and the cathode is connected to the negative terminal, the OLED is said to be forward-biased. Positive charge carriers (holes) are injected from the anode into the EL medium structure, and negative charge carriers (electrons) are injected from the cathode. Such charge carrier injection causes current flow from the electrodes through the EL medium structure. Recombination of holes and electrons within a zone of the EL medium structure results in emission of light from this zone that is, appropriately, called the light-emitting zone or interface. The organic EL medium structure can be formed of a stack of sublayers that can include small molecule layers or polymer layers. Such organic layers and sublayers are well known and understood by those skilled in the OLED art.
The emitted light is directed towards an observer, or towards an object to be illuminated, through the light transmissive electrode. If the light transmissive electrode is between the substrate and the light emissive elements of the OLED device, the device is called a bottom-emitting OLED device. Conversely, if the light transmissive electrode is not between the substrate and the light emissive elements, the device is referred to as a top-emitting OLED device. In any embodiment, however, power is supplied to the electrodes either directly through electricity-carrying busses or through thin-film electronic components powered by such busses. Moreover, in a typical display device, the light emission from an OLED light-emitting element (pixel) varies with time. Since the current necessary to drive the OLED is supplied through the busses, any limitation in the conductivity, capacitance, or inductance of the busses will limit the light emission and switching speed of the pixels.
The OLED materials emit light in proportion to the density of current passed through them. Unfortunately, the OLED materials also age and become less efficient as current is passed through the OLED and light is emitted. One way known in the art to reduce the rate of aging is to reduce the current density, typically by increasing the size of the light-emitting area, sometimes known as the aperture ratio or fill factor. However, the maximum fill factor is limited by the presence of conductive busses and thin-film electronic components, particularly for bottom-emitting devices.
Referring to FIG. 2, a bottom-emitting OLED known in the prior art is illustrated having a transparent substrate 10. Over the substrate 10, a semiconducting layer is formed providing thin-film electronic components 30 for driving an OLED. Components 30 are connected to current and signal distribution busses 19. An interlayer insulating and planarizing layer 32 is formed over the thin-film electronic components 30 and busses 19, and a patterned transparent electrode 12 defining OLED light-emissive areas 50 is formed over the insulating layer 32. An inter-pixel insulating film 34 separates the elements of the patterned transparent electrode 12. One or more first layers 14 of organic material, one of which emits light, are formed over the patterned transparent electrode 12. A reflective second electrode 16 is formed over the one or more first layers 14 of organic material. A gap separates the reflective second electrode 16 from an encapsulating cover 20. The encapsulating cover 20 may be coated directly over the reflective electrode 16 so that no gap exists. The thin-film electronic components 30 are driven by current and signal distribution busses 19 provided between light emissive areas 50 to conduct electrical power and signals from external device controllers (not shown) to the electrodes 12 and 16. However, since busses 19 are positioned between light emissive areas 50, the size and conductivity of busses 19 is limited by the desired aperture ratio of the emissive area, limiting the amount of current and switching rate of the OLED device.
Referring to FIG. 3, a top view of a simplistic, prior-art layout on a substrate 10 includes an emissive area 50, thin-film electronic components 30 for driving the electrodes, and signal and current busses 19 for providing power and signals to the thin-film electronic components 30. The relative sizes and spacing of the various elements in the device is typically defined by the requirements of the manufacturing process; this example is illustrative only and presumes that the resolution and spacing requirements of the various components is constant. The manufacturing process may define, for example, the resolution and spacing of the light-emitting area 50, the busses 19, and the size of the thin-film electrical components 30. If the size of the busses 19 is increased, thereby improving the signal and power distribution in the device, the size of the light-emitting areas 50 is decreased, thereby increasing the current density of the driving currents in the OLED (at a constant brightness) and reducing the lifetime of the materials. If the size of the light-emitting areas 50 is increased, thereby decreasing the current density of the driving currents in the OLED (at a constant brightness) and increasing the lifetime of the materials, the remaining area for the busses 19 is decreased, thereby reducing the effectiveness of the signal and power distribution in the device.
There is a need, therefore, for an improved OLED device structure that improves the power and signal distribution over the OLED device without decreasing the lifetime of the OLED materials in the OLED device.